Semiconductor device and manufacturing method thereof

ABSTRACT

A method of manufacturing a semiconductor device includes: preparing a semiconductor element having a first metal layer made of first metal on a surface thereof, and a metal substrate made of second metal, the metal substrate having a fourth metal layer made of fourth metal on a surface thereof, and mounting the semiconductor element on the surface thereof; providing metal nanopaste between the first metal layer and the fourth metal layer, the metal nanopaste being formed by dispersing fine particles made of third metal with a mean diameter of 100 nm or less into an organic solvent; and heating, or heating and pressurizing the semiconductor element and the metal substrate between which the metal nanopaste is provided, thereby removing the solvent. Further, each of the first, third and fourth metals is made of any metal of gold, silver, platinum, copper, nickel, chromium, iron, lead, and cobalt, an alloy containing at least one of the metals, or a mixture of the metals or the alloys. By the manufacturing method, it is possible to bond the semiconductor element to the metal substrate favorably.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a Divisional of U.S. application Ser. No.11/334,391, filed Jan. 19, 2006, which is based upon and claims thebenefit of priority from prior Japanese Patent Application No.2005-12333, filed Jan. 20, 2005, the entire contents of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device and amanufacturing method thereof.

2. Description of the Related Art

A technology for bonding two members by heating and baking metalnanoparticles in place of the conventional solder is described inJapanese Patent Laid-Open Publication No. 2004-128357.

Moreover, in the following document, surfaces of copper test pieces arebonded to each other by using silver nanopaste formed of silvernanoparticles coated with an organic solvent, a bonding strength of thetest pieces is measured, and a cross-sectional texture of the bondedportion is observed (refer to “The Novel Bonding Process Using AgNanoparticles”, Collection announced on Mate 2004, p. 213).

SUMMARY OF THE INVENTION

As a method of bonding a semiconductor element on a metal substrate, amethod using a variety of solders is used in general, and besides, amethod using conductive paste, brazing, and the like are also examined.However, the solder and the conductive paste are low in thermalresistance, and in the brazing, a residual stress of the bonded portionafter the bonding is large, leading to low reliability. Moreover, forthe bonding technology using the metal nanoparticles in theabove-described Publication and document, detailed examination has notbeen made yet. As described above, a satisfactory bonding method of thesemiconductor element to the metal substrate has not been realized yet.

It is an object of the present invention to provide a semiconductordevice and a manufacturing method thereof, which are capable offavorably bonding the semiconductor element to the metal substrate.

The first aspect of the present invention provides a method ofmanufacturing a semiconductor device comprising: preparing asemiconductor element having a first metal layer made of first metal ona surface thereof, and a metal substrate made of second metal, the metalsubstrate having a fourth metal layer made of fourth metal on a surfacethereof, and mounting the semiconductor element on the surface thereof;providing metal nanopaste between the first metal layer and the fourthmetal layer, the metal nanopaste being formed by dispersing fineparticles made of third metal with a mean diameter of 100 nm or lessinto an organic solvent; and heating, or heating and pressurizing thesemiconductor element and the metal substrate between which the metalnanopaste is provided, thereby removing the solvent, wherein each of thefirst, third and fourth metals is made of any metal of gold, silver,platinum, copper, nickel, chromium, iron, lead, and cobalt, an alloycontaining at least one of the metals, or a mixture of the metals or thealloys.

The second aspect of the present invention provides a semiconductordevice comprising: a semiconductor element having a first metal layermade of first metal on a surface thereof; a metal substrate made ofsecond metal, the metal substrate having a fourth metal layer made offourth metal on a surface thereof, and mounting the semiconductorelement on the surface thereof; and a bonding layer which bonds thefirst metal layer and the fourth metal layer to each other, the bondinglayer being provided between the first metal layer and the fourth metallayer, wherein each of the first, third and fourth metals is made of anymetal of gold, silver, platinum, copper, nickel, chromium, iron, lead,and cobalt, an alloy containing at least one of the metals, or a mixtureof the metals or the alloys.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the accompanyingdrawings wherein;

FIG. 1A is a cross-sectional view showing a semiconductor element and ametal substrate according to a first embodiment of the present inventionbefore being bonded to each other;

FIG. 1B is a cross-sectional view showing the semiconductor element andthe metal substrate according to the first embodiment of the presentinvention after being bonded to each other;

FIG. 2A is a cross-sectional view showing a semiconductor element and ametal substrate according to a second embodiment of the presentinvention before being bonded to each other;

FIG. 2B is a cross-sectional view showing the semiconductor element andthe metal substrate according to the second embodiment of the presentinvention after being bonded to each other;

FIG. 3A is a cross-sectional view showing a semiconductor element and ametal substrate according to a third embodiment of the present inventionbefore being bonded to each other;

FIG. 3B is a cross-sectional view showing the semiconductor element andthe metal substrate according to the third embodiment of the presentinvention after being bonded to each other;

FIG. 4A is a cross-sectional view showing a semiconductor element and ametal substrate according to a fourth embodiment of the presentinvention before being bonded to each other;

FIG. 4B is a cross-sectional view showing the semiconductor element andthe metal substrate according to the fourth embodiment of the presentinvention after being bonded to each other;

FIG. 5A is a cross-sectional view showing a semiconductor element and ametal substrate according to a fifth embodiment of the present inventionbefore being bonded to each other; and

FIG. 5B is a cross-sectional view showing the semiconductor element andthe metal substrate according to the fifth embodiment of the presentinvention after being bonded to each other.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Embodiments of the present invention are described below in detail byusing the drawings. Note that the same reference numerals are assignedto components having the same functions, and a duplicate descriptionthereof is omitted.

First Embodiment

FIG. 1A shows a semiconductor element and a metal substrate before beingbonded to each other in a first embodiment of the present invention, andFIG. 1B shows the semiconductor element and the metal substrate afterbeing bonded to each other.

A manufacturing method of this embodiment is a method of bonding asemiconductor element 1 and a metal substrate 2 to each other by usingmetal nanopaste 3. Here, the semiconductor element (semiconductor chip)1 is one of a surface mounting type, which has a first metal layer 11formed of first metal on at least one of two main surfaces opposite toeach other. The metal substrate 2 is one which is formed of second metaland mounts the semiconductor element 1 on a surface thereof. Moreover,the metal nanopaste 3 is one in which ultra fine particles formed ofthird metal with a mean diameter of 100 nm or less are dispersed into anorganic solvent. Then, each of the first, second and third metals ismade of any metal of gold (Au), silver (Ag), platinum (Pt), copper (Cu),nickel (Ni), chromium (Cr), iron (Fe), lead (Pb), and cobalt (Co), analloy containing at least one of these metals, or a mixture of thesemetals or alloys (a mixture of gold particles and silver particles, amixture of silver particles and copper alloy particles, and the like).

A description is made below in detail of the manufacturing method of asemiconductor device and a configuration of the semiconductor device.

<Preparation of Members>

First, the semiconductor element 1, the metal nanopaste 3, and the metalsubstrate 2 are prepared. One made of silicon (Si) can be used as thesemiconductor element 1. On a back surface of the semiconductor element1, a titanium (Ti) layer for making an ohmic connection between thesemiconductor element 1 and the metal substrate 2 is formed, a nickellayer for preventing a diffusion of a different type of metal into thesemiconductor element 1 is formed thereon, and a silver layer serving asthe first metal layer 11 is finally formed thereon. The first metallayer 11 can be formed on the semiconductor element 1 by a vacuumevaporation method or an electrolytic deposition method. Note that thetitanium layer and the nickel layer are not shown. Moreover, in thisembodiment, the metal substrate 2 is made of copper.

In this embodiment, the third metal constituting the metal nanopaste 3is made of silver. The silver nanopaste is formed of silver particleswith a particle diameter of approximately 10 nm, and is one of a pasteform in which the silver particles are dispersed into the solvent in astate where peripheries of the silver particles are further coated witha protection film formed of an organic material. In the case of heatingthe silver nanopaste, when the nanopaste reaches a certain temperature,the solvent and the organic protection film are decomposed andvolatilized, and surfaces of the silver as the ultra fine particlesappear. The silver nanopaste is made to function as a bonding materialby using such a principle that the ultra fine particles are sinteredtogether.

Although there is some difference depending on materials, as a basicfeature of the metal nanoparticles, the particles at such a nanolevelare agglomerated and sintered together at a temperature lower than amelting point of bulk thereof owing to surface energy thereof.Specifically, the silver nanopaste is an adhesive using a phenomenonthat, though the ultra fine silver particles usually exist stably in thesolvent without being bonded together, the silver particles which areultra fine are sintered together when the organic material isvolatilized by a heat treatment.

Note that a mean diameter of the third metal in the metal nanopaste foruse in the present invention differs depending on the melting point ofthe third metal. Specifically, in the case of metal in which the meltingpoint is low, the metal is agglomerated and sintered together at a lowtemperature even if a mean diameter thereof is large. However, in thecase of metal in which the melting point is high, a mean diameterthereof must be reduced in order to agglomerate and sinter the metal ata low temperature. From this viewpoint, when the third metal is one ofthe above-described metals, it is necessary that the mean diameter be100 nm or less. When the third metal is silver, the third metal with aparticle diameter of approximately 10 nm exerts the above-describedeffect best.

<Manufacturing Method of Semiconductor Device>

Next, the bonding is performed by using the respective members.

First, on a predetermined surface of the metal substrate 2 made of Cu,on which the semiconductor element 1 is mounted, the silver nanopaste 3is coated with a uniform thickness by using a screen printing method.

Thereafter, the semiconductor element 1 on the back surface of which thefirst metal layer 11 made of Ag is formed is disposed on the metalsubstrate 2 so that the back surface thereof can be adhered to thesilver nanopaste 3, followed by heating. In such a way, the top surfaceof the first metal layer 11 made of Ag on the back surface of thesemiconductor element 1, the top surfaces of the silver nanoparticles asthe third metal, and the top surface of the metal substrate 2 made of Cuare reduced by carbon contained in the organic material constituting thesilver nanopaste 3. Then, by agglomeration of the silver nanoparticlesin which the surfaces are reduced, the semiconductor element 1 and thesilver particles start to be bonded to each other, and the metalsubstrate 2 and the silver particles start to be bonded to each other.As a result, as shown in FIG. 1B, a semiconductor device, in which thesemiconductor element 1 and the metal substrate 2 are bonded to eachother by a bonding layer 4 made of Ag, is obtained.

Note that, with regard to a temperature at the time of theabove-described heating, it is necessary to perform the heating untilthe organic solvent and the protection film in the metal nanopaste 3 aredecomposed by heat. Specifically, it is appropriate to perform theheating at approximately 300° C. Moreover, at the time of the heating,the semiconductor element 1 and the metal substrate 2 may be pressurizedin a vertical direction with respect to the bonded surfaces thereof.Thus, water and carbon dioxide, which are generated when the organicmaterial is decomposed, can be removed from the metal nanopaste 3, andthe bonding layer 4, which is dense, is generated. In the case ofperforming the pressurization, pressurization with a pressure of severalhundreds kPa to several MPa is appropriate. However, the pressurizationis not an essential requirement in the manufacturing method of thepresent invention.

<Reason for Limiting Metallic Element>

Here, each of the first, second and third metals is made of any metal ofgold, silver, platinum, copper, nickel, chromium, iron, lead, andcobalt, the alloy containing at least one of these metals, or themixture of these metals or alloys.

It is possible to explain the material, which is reducible by the carboncontained in the organic material constituting the metal nanopaste 3, bystability of an oxide thereof. The stability can be derived by standardfree energy of formation of the oxide in each substance. Specifically, asubstance having numerically larger energy of formation at a certaintemperature has a larger affinity for oxygen, and forms a more stableoxide. Hence, when two substances to be oxidized exist, the substancehaving numerically larger energy of formation is considered to functionas a reducer while the other substance is being oxidized.

Actually, with reference to a standard free energy offormation-temperature diagram of oxides (refer to “Metal Data Book”,3^(rd) edition, p. 96, edited by The Japan Institute of Metals) and anoxidation-reduction equilibrium diagram (refer to “High TemperatureOxidation of Metals”, p. 10, written by Saitoh, Y., et al., issued byUchida Rokakuho Pub.), which are generally known in the field ofmetallurgical engineering, carbon (C) can be a reducer at approximately300° C. for an oxide of copper (CuO, Cu₂O), an oxide of chromium (CrO₃),and the like. Moreover, it is understood that carbon can also be areducer for an oxide of nickel (NiO) at around approximately 500° C.Gold, silver, and platinum are defined to be materials originallydifficult to be oxidized.

Hence, in the present invention, gold, silver, and platinum can be usedas the metals difficult to be oxidized, and copper, nickel, chromium,iron, lead, cobalt can be used as general metals, which are reducible bythe organic material and free from serious problems in particular foruse and in terms of the environment. However, it is preferable for thefirst metal to contain any metal of gold, silver, platinum, and copper,or the alloy containing at least one of these metals from a viewpoint offacilitating the coating thereof on the semiconductor device and thereduction thereof.

As described above, as the method of bonding the semiconductor elementto the metal substrate, a method using a variety of solders is used ingeneral, and besides, a method using conductive paste, brazing, and thelike have been examined. However, a melting point of the conventionaleutectic solder (60% Sn-40% Pb) is approximately 183° C., and a meltingpoint of the conventional high temperature solder (5% Sn-95% Pb) isapproximately 300° C., and an operating temperature of a semiconductordevice subjected to a bonding process using these solders has beenlimited to approximately 183° C. or less. Moreover, even in the case ofonly using the high temperature solder in order to raise the operatingtemperature, since the entire bonding must be completed by one step, anda degree of difficulty thereof is high, such concerns as an adverseeffect to yield and a cost increase of the product have remained. Evenif the above-described concerns have been solved by some measures, sucha problem on reliability remains that crystal grains of the solder areenlarged by the use thereof at the high temperature, leading to an easyoccurrence of a crack. A similar thing can also be said about lead-freesolder.

The conductive paste is a bonding material capable of performing thebonding at the low melting point and keeping bondability thereof (beingusable) at the high temperature. The conductive paste is also capable ofreducing a residual stress of the bonded portion. However, it is generalthat the maximum usable temperature of the conductive paste depends onresin functioning as the bonding material, and for example, the maximumusable temperature has such limitations as 300° C. under the operationfor several hours. Moreover, a point that adhesion force of theconductive paste is weak since strength of the bonded portion thereby islower than that by the solder is regarded as a problem. In addition,there has been a problem that bonding property of the conductive pasteis unstable, leading to an easy occurrence of variations inelectric/thermal properties.

In the brazing, a melting point of a brazing material is high.Specifically, a melting point of an Ag—Cu brazing material isapproximately 780° C., and a melting point of an Al—Si brazing materialis approximately 580° C. Accordingly, the residual stress of the bondedportion is increased, and a crack has occurred in the bonded portion orthe element itself in a cooling step after the bonding. Moreover, evenif the bonding itself is performed without trouble, an adverse effect onlong-term reliability of the semiconductor device is inevitable becauseof the above-described large residual stress.

As opposed to this, it is said that the bonding method of JapanesePatent Laid-Open Publication No. 2004-128357 is a bonding method, whichis capable of performing the bonding at the low melting point andkeeping the bondability at the high temperature, and capable ofobtaining high bonding strength equivalent to the case of using thesolder or more, thereby being excellent also in the electric/thermalproperties. However, detailed examination regarding bonding property ofmaterials to be bonded and the metal nanoparticles has not beenimplemented yet, and there is no prospect of practical use of the methodconcerned under the actual circumstances. For example, in the case ofbonding a semiconductor bare chip to an aluminum substrate by the metalnanoparticles, it is necessary to form a metal layer on the chip inadvance in consideration of the bonding property of the metal layer tothe metal nanoparticles. Simultaneously, since a strong oxide filmexists on the top surface of the aluminum, it is necessary to give somesurface treatment also to the aluminum substrate. Heretofore, theabove-described point has not been examined.

According to the manufacturing method of this embodiment, the bondedportion of the semiconductor element 1 and the metal substrate 2 has abonding structure composed of the first metal constituting the firstmetal layer 11 on the surface of the semiconductor element 1, the secondmetal constituting the metal substrate 2, and the third metalconstituting the ultra fine particles of the metal nanopaste 3. Each ofthese metals is composed of gold, silver, or platinum, which isdifficult to be oxidized, any metal of copper, nickel, chromium, iron,lead, and cobalt, which is easy to be reduced by carbon contained in theorganic solvent, the alloy containing at least one of these metals, orthe mixture of these metals or alloys. Therefore, the oxide on the topsurface of the metal concerned is reduced by the organic material, andthe respective metals can be bonded by pure surfaces thereof. Hence,such strong surface mounting, which is the best also in terms of theelectric/thermal properties, can be achieved.

Moreover, in the metal nanopaste 3, when the organic solvent and theprotection film are decomposed and volatilized at a certain temperature,the respective metal nanoparticles made of the third metal are broughtinto direct contact with one another, and the sintering at the lowtemperature, which is intrinsic to the nanoparticles, is started. Then,the bonding layer 4 made of the third metal is formed, which bonds thefirst metal on the surface of the semiconductor element 1 and the secondmetal constituting the metal substrate 2 to each other. Therefore, inthe case of using the metal nanopaste 3, the bonding can be performed ata relatively low temperature, and in addition, the metal nanopaste 3 cankeep the bondability at a temperature equal to the above-described lowtemperature or more, for example, up to a melting point of the thirdmetal staying in a bulk state. This means that this bonding material canbe used any number of times for the same part, and the conventionalprocess using the high temperature solder and the eutectic solder in twosteps is replaceable by use of only the same metal nanopaste.

Second Embodiment

FIG. 2A shows a semiconductor element and a metal substrate before beingbonded to each other in a second embodiment of the present invention,and FIG. 2B shows the semiconductor element and the metal substrateafter being bonded to each other. In FIGS. 2A and 2B, reference numeral21 denotes a fourth metal layer made of fourth metal, on which thesemiconductor element 1 is surface mounted.

A different point of the semiconductor device of this embodiment fromthe semiconductor device of the first embodiment is that, when thesecond metal constituting the metal substrate 2 is not the metal limitedin the above description, for example, when the second metal is made ofpure aluminum, the fourth metal layer 21 made of the metal limited inthe above description, for example, made of silver is formed on such anAl metal substrate 2 by using plating, vapor deposition, and the like.Since the pure aluminum has such properties as small deformationresistance and high ductility, the pure aluminum can bring an effect ofabsorbing the stress caused in a bonded portion thereof to a differentmaterial by a difference in coefficient of thermal expansion from thedifferent material. However, in the case of attempting the bonding whileusing the pure aluminum as the second metal and using the materialsaccording to the first embodiment as the other materials, since thestrong oxide film stably exists on the top surface of the aluminum, itis extremely difficult to bond the metal substrate to the semiconductorelement by using the silver nanoparticles. Hence, the surface of thealuminum is subjected to Ag plating in advance, thus making it possibleto solve the above-described problem, and to obtain an equivalent effectto that of the first embodiment. In addition, it is made possible toabsorb the stress in the bonded portion owing to the pure aluminum.

Here, besides the pure aluminum, an aluminum alloy can also be used asthe second metal. Moreover, while it is necessary that the fourth metalbe metal bondable to the third metal, specifically, the fourth metal iscomposed of any metal of gold, silver, platinum, copper, nickel,chromium, iron, lead, and cobalt, the alloy containing at least one ofthese metals, or the mixture of these metals or alloys. However, it ispreferable that the fourth metal be made of any metal of gold, silver,platinum, and copper, or an alloy containing at least one of thesemetals from a viewpoint of easiness in coating thereof on the metalsubstrate 2 and easiness in reduction thereof.

<Manufacturing Method of Semiconductor Device>

In a manufacturing method of this embodiment, first, as in the firstembodiment, the first metal layer 11 made of the first metal is formedon at least one main surface of the semiconductor element 1. Moreover,on the surface of the metal substrate 2, the fourth metal layer 21 madeof the fourth metal is formed by using a method such as plating.

Next, on a predetermined surface of the fourth metal layer 21, on whichthe semiconductor element 1 is mounted, the silver nanopaste 3 is coatedwith a uniform thickness by using the screen printing method.Thereafter, as in the first embodiment, the semiconductor element 1 onwhich the first metal layer 11 is formed is disposed so that the firstmetal layer 11 can be adhered to the silver nanopaste 3, followed byheating. In such a way, the top surface of the first metal layer 11 madeof Ag on the back surface of the semiconductor element 1, the topsurfaces of the silver nanoparticles as the third metal, and the topsurface of the fourth metal layer 21 made of the fourth metal arereduced by the carbon contained in the organic material constituting thesilver nanopaste 3. Then, by agglomeration of the silver nanoparticlesin which the surfaces are reduced, the semiconductor element 1 and thesilver particles start to be bonded to each other, and the fourth metallayer 21 and the silver particles start to be bonded to each other. As aresult, as shown in FIG. 2B, a semiconductor device, in which thesemiconductor element 1 and the fourth metal layer 21 are bonded to eachother by the bonding layer 4 made of Ag, is obtained.

As described above, according to the manufacturing method of thisembodiment, the bonded portion of the semiconductor element 1 and themetal substrate 2 has a bonding structure by the first metalconstituting the first metal layer 11, the fourth metal constituting thefourth metal layer 21, and the third metal in the metal nanopaste 3.Accordingly, a similar effect to that of the first embodiment can beobtained. In addition, such metal as the aluminum and the aluminumalloy, which are easy to be deformed by a small stress, can be used asthe second metal constituting the metal substrate 2, and accordingly, itis made possible to absorb the stress caused by the difference incoefficient of thermal expansion between the different materials. Thestress cannot be absorbed only by the first, third and fourth metals.

Third Embodiment

FIG. 3A shows a semiconductor element and a metal substrate before beingbonded to each other in a third embodiment of the present invention, andFIG. 3B shows the semiconductor element and the metal substrate afterbeing bonded to each other. In FIGS. 3A and 3B, reference numeral 22denotes a first interposed layer interposed between the metal substrate2 and the fourth metal layer 21 on which the semiconductor element 1 issurface mounted.

A different point of a semiconductor device of this embodiment from thesemiconductor device of the second embodiment is that the firstinterposed layer 22 is provided as a barrier layer between the metalsubstrate 2 and the fourth metal layer 21 on the surface thereof. Forexample, when the metal substrate 2 is made of aluminum, and the fourthmetal layer 21 is made of silver, there is a risk that the aluminum andthe silver are mutually diffused. Accordingly, this embodiment has aconfiguration in which the first interposed layer 22 made of Ni or Ti isinterposed between the second metal constituting the metal substrate 2and the fourth metal constituting the fourth metal layer 21 in order toprevent the mutual diffusion of both thereof. Thus, more reliablebonding can be ensured.

<Manufacturing Method of Semiconductor Device>

In a manufacturing method of this embodiment, first, as in the firstembodiment, the first metal layer 11 made of the first metal is formedon at least one main surface of the semiconductor element 1. Moreover,on the metal substrate 2 made of pure aluminum, the first interposedlayer 22 is formed by the method such as plating. Then, on the surfaceof the metal substrate 2, on which the first interposed layer 22 isformed, the fourth metal layer 21 is further formed by the method suchas plating.

Next, on a predetermined surface of the fourth metal layer 21, on whichthe semiconductor element 1 is mounted, the metal nanopaste 3 is coatedwith a uniform thickness by using the screen printing method.Thereafter, as in the first embodiment, the semiconductor element 1 onwhich the first metal layer 11 is formed is disposed so that the firstmetal layer 11 can be adhered to the metal nanopaste 3, followed byheating. As a result, as shown in FIG. 3B, a semiconductor device, inwhich the semiconductor element 1 and the fourth metal layer 21 arebonded to each other by the bonding layer 4, is obtained.

As described above, according to the semiconductor device and themanufacturing method thereof in this embodiment, the first interposedlayer 22 exists as the barrier layer between the second metalconstituting the metal substrate 2 and the fourth metal layer 21 formedon the metal substrate 2. Accordingly, such diffusion reactions of therespective metals can be prevented, and reliable bonding can be ensured.

Fourth Embodiment

FIG. 4A shows a semiconductor element and a metal substrate before beingbonded to each other in a fourth embodiment of the present invention,and FIG. 4B shows the semiconductor element and the metal substrateafter being bonded to each other. In FIGS. 4A and 4B, reference numeral12 denotes an electrode of the semiconductor element 1, and referencenumeral 13 denotes a second interposed layer interposed between theelectrode 12 and the first metal layer 11 formed on the lower surface ofthe semiconductor element 1.

A different point of a semiconductor device of this embodiment from thesemiconductor device of the third embodiment is that not the backsurface of the semiconductor element 1 but the surface of thesemiconductor element 1, on which the electrode exists, is bonded to themetal substrate by using the metal nanopaste. In general, on the surfaceof the semiconductor element 1, the aluminum electrode 12 made of fifthmetal, for example, of pure aluminum exists. If the first metal layer 11made of silver is directly provided on the aluminum electrode 12, thealuminum and the silver may be mutually diffused. Accordingly, thisembodiment has a configuration in which the second interposed layer 13made of Ni or Ti is interposed between the fifth metal constituting theelectrode 12 and the first metal constituting the first metal layer 11in order to prevent the mutual diffusion of both thereof. Thus, morereliable bonding can be ensured. However, the second interposed layer 13is not an essential requirement in the present invention.

In addition, by using a method of this embodiment, it is made possibleto perform the bonding using the metal nanopaste for both of the surfaceand back surface of the semiconductor element 1. Specifically, therepeated bonding in the same part, which is one of the effects of thebonding using the metal nanoparticles, is possible. For example, in atransistor element and the like, such a process is also made possible,in which, after a collector electrode on the back surface of thesemiconductor element 1 is bonded to the metal at a collector potential(metal substrate on which the metal concerned is formed) by using thesilver nanopaste, an emitter electrode on the surface of thesemiconductor element 1 is bonded to a terminal of the emitter electrodeby using the same silver nanopaste.

<Manufacturing Method of Semiconductor Device>

A manufacturing method of this embodiment is described. First, on one ofthe main surfaces of the semiconductor element 1, the electrode 12 isformed by a sputtering method and the like. Then, on the surface of theelectrode 12 formed on the semiconductor element 1, the secondinterposed layer 13 is further formed by the sputtering method and thelike. Thereafter, as in the first embodiment, the first metal layer 11is formed on the second interposed layer 13.

Moreover, separately from the above, on the surface of the metalsubstrate 2, the first interposed layer 22 is formed by the method suchas plating. Then, on the surface of the metal substrate 2 on which thefirst interposed layer 22 is formed, the fourth metal layer 21 isfurther formed by the method such as plating.

Next, on a predetermined surface of the fourth metal layer 21, on whichthe semiconductor element 1 is mounted, the metal nanopaste 3 is coatedwith a uniform thickness by using the screen printing method.Thereafter, as in the first embodiment, the semiconductor element 1 onwhich the first metal layer 11 is formed is disposed so that the firstmetal layer 11 can be adhered to the metal nanopaste 3, followed byheating. As a result, as shown in FIG. 4B, a semiconductor device, inwhich the semiconductor element 1 and the fourth metal layer 21 arebonded to each other by the bonding layer 4, is obtained.

Note that, in the drawings for the first embodiment, the secondembodiment, and the third embodiment, the electrodes of thesemiconductor element 1 are not shown. Moreover, in each of thestructures shown in FIG. 1B, FIG. 2B, and FIG. 3B, it is naturallypossible to provide the electrode 12, and further to interpose thesecond interposed layer 13 between the electrode 12 and the first metallayer 11 as in the fourth embodiment.

Fifth Embodiment

FIG. 5A shows a semiconductor element and a metal substrate before beingbonded to each other in a fifth embodiment of the present invention, andFIG. 5B shows the semiconductor element and the metal substrate afterbeing bonded to each other. In FIGS. 5A and 5B, reference numeral 23denotes an insulating plate.

In a semiconductor device of this embodiment, the metal substrates 2includes the insulating plate 23. The insulating plate 23 is composed ofceramics of aluminum nitride (AlN), silicon nitride (SiN), or the like.

A different point of the semiconductor device of this embodiment fromthe semiconductor device of the fourth embodiment is that the insulatingplate 23 is bonded to the metal substrate 2 made of the second metal inadvance. Specifically, for example, when the second metal is purealuminum, and the insulating plate 23 is made of the ceramics ofaluminum nitride, silicon nitride, or the like, a configuration is made,in which the two metal substrates 2 are bonded to each other whilesandwiching the insulating plate 23 therebetween. Thus, one of the metalsubstrates 2, which is on an opposite side to the metal substrate 2bonded to the semiconductor element 1 with respect to the ceramicsinsulating plate 23, is electrically insulated completely from theopposite metal substrate 2 by the insulating plate 23. Accordingly, itis also made possible to ensure insulating property of the semiconductordevice.

A manufacturing method of the semiconductor device in this embodiment isa similar method to that of the fourth embodiment. Specifically, thesemiconductor element 1 in which the electrode 12, the second interposedlayer 13, and the first metal layer 11 are provided, and the metalsubstrate 2 in which the fourth metal layer 21, the first interposedlayer 22, and the insulating plate 23 are provided, are prepared inadvance.

Next, on a predetermined surface of the fourth metal layer 21, on whichthe semiconductor element 1 is mounted, the metal nanopaste 3 is coatedwith a uniform thickness by using the screen printing method.Thereafter, as in the first embodiment, the semiconductor element 1 onwhich the first metal layer 11 is formed is disposed so that the firstmetal layer 11 can be adhered to the metal nanopaste 3, followed byheating. As a result, as shown in FIG. 5B, a semiconductor device, inwhich the semiconductor element 1 and the fourth metal layer 21 arebonded to each other by the bonding layer 4, is obtained.

Moreover, in each of the structures shown in FIG. 1B, FIG. 2B, and FIG.3B, it is naturally possible to provide the insulating plate as in thefifth embodiment.

Note that the embodiments explained above are described for facilitatingthe understanding of the present invention, and are not described forlimiting the present invention. Hence, the respective componentsdisclosed in the above-described embodiments are shown as objectsincorporating the entire design changes and equivalents, which belong tothe technical scope of the present invention. For example, though Ag,Cu, Al and the like are used as the metals in the above embodiments, thepresent invention is not limited to this, and the metals may be metalsaccording to claims and alloys thereof. In particular, though the silvernanoparticles are used as the third metal, the third metal may be metalsaccording to claims, alloys thereof, or mixture particles of these.

Moreover, though Si is used as the semiconductor element 1, thesemiconductor element 1 may be made of gallium arsenide (GaAs), siliconcarbide (SiC), and the like, which are other than Si. As the effectsobtained by the present invention, mentioned are the reduction of theresidual stress of the bonded portion, which is obtained by thelow-temperature bonding using the silver nanopaste, the relaxation oflimitations on the operating temperature of the semiconductor deviceafter the bonding, and the absorption of the stress by the aluminumsubstrate. As a usage purpose in which the above-described effects canbe effectively utilized, mentioned is a mounting method forhigh-temperature use of SiC promising as a highly heat-resistantelement.

The entire content of a Japanese Patent Application No. P2005-012333with a filing date of Jan. 20, 2005 is herein incorporated by reference.

Although the invention has been described above by reference to certainembodiments of the invention, the invention is not limited to theembodiments described above will occur to these skilled in the art, inlight of the teachings. The scope of the invention is defined withreference to the following claims.

1. A semiconductor device, comprising: a semiconductor element having afirst metal layer made of first metal on a surface thereof; a metalsubstrate made of second metal, the metal substrate having a fourthmetal layer made of fourth metal on a surface thereof, and mounting thesemiconductor element on the surface thereof; and a bonding layer whichbonds the first metal layer and the fourth metal layer to each other,the bonding layer being provided between the first metal layer and thefourth metal layer, wherein each of the first, third and fourth metalsis made of any metal of gold, silver, platinum, copper, nickel,chromium, iron, lead, and cobalt, an alloy containing at least one ofthe metals, or a mixture of the metals or the alloys.
 2. A semiconductordevice according to claim 1, wherein each of the first and fourth metalsis any metal of gold, silver, platinum, and copper, or an alloycontaining at least one of the metals.
 3. A semiconductor deviceaccording to claim 1, wherein the second metal is same as the fourthmetal.
 4. A semiconductor device according to claim 1, wherein thesecond metal is made of pure aluminum or an aluminum alloy.
 5. Asemiconductor device according to claim 1, further comprising: a firstinterposed layer made of nickel, a nickel alloy, titanium, or a titaniumalloy, the first interposed layer being interposed between the metalsubstrate and the fourth metal layer.
 6. A semiconductor deviceaccording to claim 1, further comprising: an electrode made of fifthmetal, the electrode being provided between the semiconductor elementand the first metal layer and brought into contact with thesemiconductor element, wherein the fifth metal is made of pure aluminumor an aluminum alloy.
 7. A semiconductor device according to claim 6,further comprising: a second interposed layer made of nickel, a nickelalloy, titanium, or a titanium alloy, the second interposed layer beinginterposed between the electrode and the first metal layer.
 8. Asemiconductor device according to claim 1, further comprising: aninsulating plate which electrically insulates one surface of the metalsubstrate and a surface thereof opposite to the one surface from eachother.